1. Field of the Invention
The present invention relates to a circuit for rapid recognition of frequency shift keyed (FSK) signals in a radio channel given pre-emphasis and de-emphasis.
2. Description of the Prior Art
A phase locked loop (PLL) is generally employed for detecting the FSK signal in a radio channel. With the use of a quadrature detector as the locked detector and with integration at the output, the circuit emits an output signal when it has been locked onto an input signal. The criterion for the lock-on is that the input signal lies above a specific value and, moreover, it is located within a specific frequency band around the center frequency of the phase locked loop. This frequency band is thereby fixed by the loop amplification and by the loop attenuation factor of the phase locked loop which are, in turn, determined by the baud value of the frequency deviation of the FSK data signal.
When no radio frequency signal is received by the antenna input of a radio receiver, a white noise will occur at the audible output in accordance with the thermal noise generated in the receiver when no filtration occurs. In most frequency modulation (FM) radio systems, the audible output is filtered with a low-pass filter of the first order with a frequency cutoff at a few 100 Hz, the so-called de-emphasis. In the transmitter, the audible signal is filtered with a high-pass filter, the so-called pre-emphasis, in order to obtain a flat signal curve in the overall system.
The de-emphasis in the receiver is usually the cause of the pink noise energy (i.e. +6 dB/octave) in the demodulated data channel when no radio frequency signal is received by the antenna input of the receiver. As a result, the phase locked loop is locked onto the noise for a rather long period. Unless the lock detector output is integrated over a rather long period, this will cause an erroneous lock-on to the noise.
In specific instances, a long integration time can be unacceptable in a radio system because the time available for transferring from one channel to the next is very short.